Aspergillus is a saprophytic bacterium that is widely found in nature and is a resident fungus of normal human skin mucosa. Aspergillus spores are small, have a diameter of 2-3 μm, can float in the air, and enter the human body through the respiratory tract. As the Aspergillus enters the body mainly through the respiratory tract, the Aspergillus infection mainly occurs in the lungs.
The incidence of invasive Aspergillosis (IA) in immunosuppressed patients is increasing year by year due to the abuse of antibiotics and is the main cause of death. Aspergillus fumigatus is the most common pathogen causing severe deep Aspergillus infection in immunosuppressed patients, followed by Aspergillus flavus, Aspergillus niger, and Aspergillus terreus, etc. IA has a mortality rate as high as 70%-90% in patients with hematonosis and hematopoietic stem cell transplants (HSCT). The main reason for this high mortality rate is that IA cannot be effectively detected and diagnosed at the early stage of the course of disease, causing patients to die without timely and effective treatment. Therefore, it is of great significance to select early detection and diagnosis methods.
Currently, the widely recognized methods for detecting Aspergillus antigen mainly include 1,3-β-D glucan detection (G test) and galactomannan test (GM test). 1,3-β-D glucan antigen is a specific cell wall component of all fungi except the tubercle bacillus and Cryptococcus, with serum as a test sample, its sensitivity and specificity may reach 80%. However, because it is negative for colonized Candida, it often needs to be combined with the GM test for exclusion, when both are negative, fungal infection can be basically ruled out. In addition, the serological G test is susceptible to hematology and other factors such as fibrous substances, etc. Moreover, 1,3-β-D glucan antigen may form an immune complex with antibodies in the blood, which is rapidly cleared by the blood, resulting in a false negative.
Galactomannan is a highly specific and highly conserved polysaccharide present in cell wall of Aspergillus, which can be used as a specific molecular marker for detection of Aspergillus. Enzyme-linked immunosorbent assay (ELISA) is a relatively common method for detecting galactomannan. Acosta J reported that GM positive results were 4.3 days earlier than Aspergillus culture. A positive serum GM test is appropriate and applicable for the diagnosis of invasive fungal infections, and is further an important hint for patients taking early antifungal therapy, especially for some high-risk patients (such as HSCT patients). Therefore, determination of GM antigen levels in serum contributes to early diagnosis and early treatment of IA.
At present, the Aspergillus GM detection kits on the market have low sensitivity, specificity and sensitivity in detection, e.g., the sensitivity is about 83%, the specificity is 90%, and the detection limit is about 1 μg/L. They generally adopt a double antibody sandwich-enzyme linked immunosorbent assay, that is, first, a specific monoclonal antibody of Aspergillus is coated on an ELISA plate, and then a sample to be tested and the same monoclonal antibody labeled with horseradish peroxidase (HRP) are added, the antigen in the sample to be tested will bind to the specific monoclonal antibody and form a sandwich structure, then a color developing agent is further added for color development, and the depth of the color is positively correlated with the concentration of the antigen to be tested, thereby realizing the detection of GM antigen. The method has tedious detection steps, and high preparation cost of monoclonal antibody, which is not conducive to the promotion and popularization of the kits in clinical detection.